AzOcean, I am not sure if all of them will stay at the surface though, so it may be hard to tell when they died, if your sand bed is dep like mine.
Also, I was wondering,.... I doubt that they are actually "eating the nitrates", I think it is more along the lines that...
If you have enough flow, then most detritus, will stay "water borne". The clams filter the water (actually, I am thinking that it would be very strange, that all clams that we get as "cleaner" clams, actually filter out of the water column, it would seem that some would sift sand), which is laden with detritus. While filtering the water, they ingest this detritus, which means that their is now less detritus in the water, less detritus = less decomposing matter= less nitrates.
Please correct me if I am bieng crazy AGAIN!